Geothermal heating and cooling systems typically employ water as a medium to absorb and retain heat. The water that passes through the geothermal heating piping system can be passed through a heat exchanger, to which a blower is attached to pass the air over the heat exchanger.
The geothermal heating and cooling system obtains its ability to aid in the heating or cooling of a building by exploiting the general constancy of ground temperatures. A typical geothermal heating system comprises a closed loop pipe system through which water is pumped. A portion of the pipe is disposed underground. Often a bore hole is drilled into the ground into which a portion of the closed loop is placed. As the water in the pipe travels in the pipe down and up the bore hole, the temperature of the ground surrounding the bore hole serves to either add heat to the water in the pipe or absorb heat from the water in the pipe, depending upon whether the water within the pipe is hotter or cooler than the surrounding ground temperature. Since the ground surrounding a bore hole remains at a generally constant temperature, the water passing through the pipe can, at least theoretically can be heated or cooled to this constant temperature regardless of the season. This enables the geothermal system to deliver water for use at the building that is generally at the same temperature on a year-round basis.
In most cases, geothermal heating and cooling systems are used in connection with a mechanical refrigeration system, such as a heat pump.
A typical mechanical refrigeration system includes a pair of heat exchangers, and a closed-loop piping system that runs between, and in both of the two heat exchangers. A refrigerant, such as R-22-type refrigerant is passed through the mechanical refrigeration systems' piping system. A compressor is provided for compressing the refrigerant from a gaseous to a liquid state, and an expansion valve is provided for enabling the liquified refrigerant to expand from a compressed, liquid state into a gaseous state. As is well known within the refrigeration art, the expansion of a refrigerant from a liquid to a gaseous state absorbs heat, whereas the compression of a refrigerant from a gaseous to a liquid state gives off heat.
In a typical heat pump system, a first heat exchanger is placed just downstream of the expansion valve, and a second heat exchanger that is disposed downstream of the compressor. The expansion of the refrigerant from a liquid to a gaseous state cools the “pipe” (which is formed into a “coil”) through which the refrigerant is flowing in the first heat exchanger. A fan can then be provided to pass air over the now-cooled pipe coil of the first heat exchanger, so that the air passed thereover becomes cooled by the pipes. When operated in the air conditioning mode, this cooled air is then circulated throughout the building and serves to cool the building. When in the air conditioning mode, this first heat exchanger is usually placed within the interior of the building.
During this same air conditioning cycle, the other heat exchanger, that is placed downstream of the compressor, is placed outside of the building. When the compressor compresses the refrigerant into a liquid refrigerant, heat is given off. A fan can be placed adjacent to the outside heat exchanger to blow air over the heat exchanger to thereby help to remove the hot air from the vicinity of the heat exchanger, and to draw cooler air into the area near the heat exchanger to absorb more heat from the heat exchanger coil.
To a large extent, the efficiency of a mechanical refrigeration system unit is determined by the ambient temperature of the air that is adjacent to the outside heat exchanger, for the more heat that can be exchanged, the more efficiently the compressor can compress the refrigerant into a liquid form.
The above passage describes the operation of a heat pump system when it is operating in an “air conditioning mode”. When the mechanical refrigeration system is operating in a “heating mode”, the roles of the two heat exchangers are reversed. As such, the interior (first) heat exchanger is placed downstream of the compressor, so that the compression of the liquid refrigerant will give off heat, to thereby heat the air that is blown past the heat exchanger by the fan. This heated air is then circulated throughout the building for heating the building. The outside heat exchanger is placed downstream from the evaporator so that, in the expansion process, it can pick up heat from the ambient environment.
When operating in the heating mode, the efficiency of the refrigeration system and its ability to heat a building is largely dependent on the efficiency by which heat is exchanged in the “outside” heat exchanger. For example, on a very cold day, when the mechanical refrigeration system is serving as a “heater”, the coldness of the outside air may provide little heat for the evaporating refrigerant in the second (outside) heat exchanger to absorb. Similarly, when used in an air-conditioning capacity during the summer, the heat of the outside air reduces the efficiency of the condenser's ability to expunge heat from the refrigerant during the compression of the refrigerant into liquid refrigerant by the compressor.
One method for improving the efficiency of such a mechanical refrigeration system is to immerse the heat exchanger in a liquid medium such as water. Use of water as a heat exchange medium helps to improve the efficiency in two ways. The first way it improves efficiency is that water is a better heat exchange medium than air.
A second manner in which efficiency can be improved by placing the water in the heat exchanger at a more appropriate temperature than the corresponding air. For example, water at 52° F. (11° C.) on a hot, 90° F. (32° C.) summer day will much more efficiently absorb heat from a hot condenser coil (outside heat exchange unit) of a mechanical refrigeration system than will the 90° F. (32° C.) ambient air. Conversely, 52° F. (11° C.) water will have a greater propensity to give off heat to an evaporator heat exchange coil on a cold, 10° F. (−12° C.) winter day, than the 10° F. (−12° C.) ambient air.
To capitalize on these efficiencies, a geothermal heating system can be coupled to a mechanical refrigeration system.
In order to prevent the pollution of aquifers, most geothermal energy systems are constructed as a closed-loop system, where water is constantly re-circulated through a closed-loop. A portion of the closed loop extends deep into the ground, so that water passing in the underground portion of the closed loop can take advantage of the relatively constant ground temperature by exchanging heat with the ground surrounding the pipe, so that the water in the geothermal pipe will emerge from the ground at a temperature approximating the ground temperature.
A typical prior art geothermal installation is schematically represented in FIG. 1. A building 10, such as a house, school, factory, office building or the like, includes a mechanical refrigeration system 12, to which the geothermal system 36 is coupled. The mechanical refrigeration system 12 includes an inside (first) heat exchanger 14 and an outside (second) heat exchanger 18. In a heat pump-type mechanical refrigeration system, the inside heat exchanger 14 serves as an evaporator when the system 12 is serving as an air conditioner, and as a condenser when a mechanical refrigeration system 12 is serving as a heating unit. Conversely, the outside heat exchanger 18 serves as a condenser when the mechanical refrigeration system 12 is being used as an air conditioner or cooler, and serves as an evaporator when the mechanical refrigeration system 12 is being used as a heater.
The inside heat exchanger 14 includes a coil 16 through which refrigerant flows, and a fan 22 for pulling air through the inside heat exchanger 14 cabinet, to move air past and over the coil 16, so that the air thus moved by will become cooled through its contact with the coil 16 when the mechanical refrigeration system 12 is being used as an air conditioner, and will become heated when the mechanical refrigeration system 12 is using the inside heat exchanger 14 as a condenser during a heating operation. The outside heat exchanger 18 also includes a coil that is part of the closed-loop of the mechanical refrigeration system. The inside and outside heat exchangers 16, 18 can be constructed generally similarly, except that the outside heat exchanger should be weatherized to withstand outside weather conditions.
An expansion valve 24 and a compressor 26 are provided for allowing the refrigerant to expand (expansion valve 24), and to compress the refrigerant (compressor 26). The outside heat exchanger includes a cabinet 28 that contains the coil 20. The cabinet 28 includes an inflow port 30 through which water from the geothermal heat exchange system 36 can enter the interior of the cabinet 28, and an outflow port 32 from which water of the geothermal exchange system 36 can exit the cabinet 28.
The geothermal exchange system 36 is shown as comprising a closed-loop pipe system 38, wherein water or other fluid within the geothermal system 36 is re-circulated. The geothermal exchange system includes an inflow pipe 40 that brings water into the cabinet 28 of the outside heat exchanger 18, and an outflow pipe 42 that carries water away from the cabinet 28 of the outside heat exchanger 18. A pump 44 is provided for pumping water through the closed-loop geothermal heating system.
The outflow pipe 42 includes a subterranean portion 46, that is disposed below ground level. The inflow pipe 40 also includes a subterranean portion 48 disposed below ground level. The subterranean portions 46, 48 of the outflow pipe 42 and inflow pipe 40 are joined at a U-shaped connector 50, so that water reaching the lower “end” of the outflow pipe 42 can flow through the connector 50 into the inflow pipe 40.
The subterranean portions 46, 48 are typically positioned within a bore hole 52. In a “vertical” geothermal system, the bore hole may be quite deep, and may often exceed 100 feet (30.5 m) in length. Bore holes of 1000 feet (305 m) in length are not rare. Typically, a bore hole of six to eight inches (15.3 cm to 20.3 cm) in diameter is employed, as a bore hole of that size will provide enough area for the insertion of both the subterranean portions 46, 48 of the inflow pipe 40 and outflow pipe 42.
After the bore hole 52 is dug, and the subterranean portions 46, 48 of the outflow pipe 42 and inflow pipe 40 are inserted into the bore hole 52, the area around the pipe is packed with a grouting material, that may comprise bentonite. The grouting is provided both for providing stability to the hole, and also to prevent water or fluid flowing through the inflow and outflow pipes 40, 42 from coming in contact with any water and any aquifers through which the pipes 40, 42 may pass.
The depth of the bore hole will vary based on a variety of factors, and several factors must be taken into consideration when determining how deep to drill the bore hole. One factor relates to cost. For the two-separate side-by-side pipe type system shown in FIG. 1 and described above, the installer must normally employ a bore hole having a six inch (15.3 cm) diameter or greater, in order to accommodate the pipes. The cost of drilling the bore hole at typical 2007 rates is somewhere between $6.00-$8.00 per foot (0.3 m). As such, a 100′ (30 m) hole would typically cost somewhere between $600.00 and $800.00 in drilling costs alone. As such, cost considerations suggest that it is preferable to drill the hole no deeper than one needs to.
The second consideration relates to heat exchange capacity. As water flowing through the subterranean portions 46, 48 of the pipe exchanges heat with the ground in which the bore hole is dug, it follows naturally that a deeper (longer) bore hole would provide a greater heat exchange capacity than a shallower (shorter) bore hole, if, for no other reason than a deeper bore hole would provide a greater residence time for water within the subterranean portions 46, 48 of a geothermal system, and would provide a greater surface area of “ground” with which to exchange heat.
In this regard, the Applicant has found, that a “ton” of heating or cooling capacity is typically achieved by a bore hole of between 150 and 200 feet (46 and 61 m) with a side-by-side pipe system. By way of example, if one desires to achieve four tons of heating and cooling capacity (heat exchange capacity), it follows that one would need to drill a bore hole that was somewhere between 600 and 800 feet (183 and 244 m) in depth.
Another factor that affects the decision of how deep to drill the bore hole (and hence, its associated cost) relates to the heat exchange capacity of the particular materials used in constructing the subterranean portions 46, 48 of the pipe, and the grout that is disposed in the space 52 between the pipes and the edge of the bore hole.
To a large extent, environmental considerations and reliability considerations impact the geothermal system constructor's ability to achieve optimum heat exchange capabilities. In theory, one could likely improve the heat exchange capabilities of the subterranean pipes 46, 48 by employing metal pipes as metals usually have a greater thermal conductivity than plastics (e.g. polybutylene piping). Unfortunately, steel and metal pipes are often unacceptable, because of their propensity to corrode, and hence fail over a reasonably short period of time. As such, reliability, cost concerns, and environmental suggest that one employ plastic pipe. Although plastic pipe has generally poorer thermal conductivity properties than metal, it is much more durable.
Environmental concerns also factor into the technologies by which one can construct a geothermal system. These environmental concerns arise largely from the fact that many cities forbid the use of “pump and dump” geothermal systems. In pump-and-dump systems, the water for the geothermal system is drawn from an aquifer, run through the heat exchanger, and then deposited back into the aquifer. Such pump and dump systems are forbidden in many locations because of the fact that they can pollute the ground water aquifer. As such, most currently-installed geothermal systems are closed-loop systems, that re-circulate the same water.
In order to protect the aquifer, it is often required that the system be sealed from the “soil” of the walls of the bore hole through the use of some impervious material that prevents water in the pipe 46, 48 from leaking into the aquifer. This impervious material typically comprises a “grout”. The grout may be made of one of a variety of materials, such as a generally impervious bentonite clay. This bentonite clay is placed in the bore hole to surround and encase the subterranean pipes 46, 48.
Unfortunately, the grout adversely impacts the heat transfer capabilities of the pipe. To overcome the adverse impact on heat transfer properties caused by the grout, the installer is forced to drill the bore hole much deeper than if the pipes 46, 48 could directly contact the soil.
One improvement to the above-mentioned dual-pipe system is a concentric pipe system invented earlier by the Applicant.
The concentric (and typically co-axial) pipe is schematically shown in FIGS. 2 and 3 as including an outer, outflow pipe 54, that preferably has a 3″ (7.6 cm) diameter, and an inflow pipe 56. The inflow pipe 56 is disposed concentrically and interiorly of the outflow pipe 54, and typically has a one or 1.25 inch (2.54 or 3.2) cm diameter.
The concentric pipe has significant benefits over the twin-pipe system shown in FIG. 1. One benefit is that it can be placed in a smaller bore hole, such as a 4″ or 4.5″ (10 or 11.5 cm) diameter bore hole, rather than the 6″ (15.25 cm) diameter bore hole typically used for the twin-pipe system shown in FIG. 1. This use of a smaller bore hole helps to reduce drilling costs, as it costs less per foot (typically $6.00 per foot versus $8.00 per foot (0.3 m) for a 6″ (15.25 cm) bore hole). Additionally, because of the configuration of the concentric pipe arrangement 54, 56, there is usually a smaller gap between the exterior wall of the outflow pipe 54, and the inner wall of the bore hole. This smaller gap requires less grout to be placed between the concentric pipe 53 and concentric pipe 58 and the bore hole wall. This use of a thinner layer of grout both helps to reduce grout costs. More importantly, a thinner grout layer permits better heat exchange between concentric pipe system 58 and the grout surrounding the bore hole.
Although the above two described configurations do perform their functions in a workman-like manner, room for improvement exists. Accordingly, it is one object of the present invention to provide an improved pipe system for use in connection with a geothermal energy system.